Rapid charge lithium ion battery charger

ABSTRACT

A rapid charging circuit for a lithium ion battery. The battery charger in accordance with the present invention compensates for the voltage drops across the various resistance elements in the battery circuit by setting the charging voltage to a level to compensate for the initial resistance of the series resistances in the circuit and an additional resistance selected to take into account the anticipated increase in resistance of the various circuit elements over time. The battery charger in accordance with the present invention periodically monitors the open-circuit voltage of the battery cell and reduces the charging voltage to when the battery cell voltage reaches the optimal value. Thus, during a constant current charging mode, the battery cell is driven at a relatively optimal charging current to reduce the charging time. As such, the system is able to optimize the charging current supplied to a battery cell during a constant current mode of operation while compensating for circuit elements whose resistance may vary over time due to temperature or other factors, such as corrosion, while at the same time avoiding exceeding the maximum recommended voltage for the battery cell.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional application of commonly-owned U.S.patent application Ser. No. 11/836,946, filed Aug. 10, 2007, which is adivisional application of U.S. patent application Ser. No. 11/241,718,filed on Sep. 30, 2005.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a battery charger and, moreparticularly, to a battery charger for rapidly and safely charginglithium ion batteries which automatically compensates for anticipatedchanges of the resistance of the battery charging circuit (i.e. externalbattery terminals, wires, and internal battery cell resistance) overtime, due to, for example, oxidation of the external battery contacts,and provides a maximum and constant current to a battery cell over theanticipated resistance range of battery charging circuit in order tominimize the charging time of the battery cell.

2. Description of the Prior Art

Battery chargers for charging lithium-ion-type batteries are known inthe art. Examples of such battery chargers are disclosed in U.S. Pat.Nos. 5,670,862; 6,002,237 and 6,859,014. Such lithium ion batterychargers are also disclosed in U.S. Patent Application Publication Nos.U.S. 2001/0011882 A1; U.S. 2003/0057920 A1 and U.S. 2003/0141850 A1; aswell as Japanese Patent No. JP 20-00277151 and Chinese Patent No. CN1269616. As is known in the art, such lithium ion batteries requireconstant current (CC) and constant voltage (CV) charging. In particular,initially such lithium ion batteries are charged with a constantcurrent. In the constant current mode, the charging voltage is typicallyset to a maximum level recommended by the LiIon cell manufacturer basedon safety considerations, typically 4.2V per cell. The charging currentis limited by the circuit to a design level, based on the cellcapability, charge time, needs and cost. Once the battery cell voltagerises sufficiently, the voltage drop across the series resistances inthe battery charging circuit forces the charging current to drop belowan initial charge current level. In particular, when the battery cellvoltage Vb approaches the charging voltage Vc, the charging currenttapers according to the formula: I=(Vc−Vb)/Rs, where I=the chargingcurrent, Vc=the charging voltage, Vb=the battery cell voltage and Rs=theresistance of the charging circuit including the external contactresistance, the resistance of the battery terminals and wires used toconnect the circuit, as well as the internal resistance of the batterycell. As such, during the last portion of the charging cycle, typicallyabout the last ⅓, the battery cell is charged at a reduced chargingcurrent, which means it takes more time to fully charge the batterycell.

In order to decrease the time to fully charge such lithium ion batterycells, many known battery chargers take into account the voltage dropacross the battery circuit elements in order to maximize the amount ofcharging current during a constant current mode. One method ofdetermining the voltage drop of the battery circuit resistance elementsis to take the difference between the closed-circuit voltage (i.e.charging voltage), and the open circuit voltage, which is the batterycell voltage with no charging current. The closed-circuit voltagerepresents the voltage of the battery cell plus the voltage drops in thecircuit as a result of resistance in the battery circuit, such as thebattery terminals and the internal resistance of the battery cell. Bysubtracting the closed-circuit voltage from the open-circuit voltage,the voltage drop across the battery resistance circuit elements can bedetermined.

Various known battery chargers use this voltage drop to drive thebattery charging voltage during a constant current mode in order toincrease the amount of current applied to the battery cell during aconstant current mode. By increasing the amount of current applied tothe battery cell during a constant current mode, the battery cell ischarged much faster.

Examples of lithium ion battery charger circuits that compensate for thevoltage drop in the battery circuit in order to increase the chargingcurrent and thus decrease the charging time for a lithium ion batteryare disclosed in U.S. Pat. Nos. 5,670,862; 6,859,014 and 6,002,237. Moreparticularly, the '862 patent teaches a compensation circuit forcompensating for predetermined voltage drop in the battery circuit. Thecompensation circuit includes an operational amplifier as well as aresistor sized to take into account the expected electrical seriesresistance of the battery circuit. The compensation circuit is based onan assumed initial voltage drop across the various resistance elementsin the circuit and compensates for this voltage drop to maintain apredetermined charging current during a constant current charging mode.Unfortunately, the resistance of the various resistance elements changeover time due to various factors including oxidation of the externalbattery contacts used to connect the battery cell to the batterycharger. The compensation circuit disclosed in the '862 patent does nottake into account such changes in resistance over time. Accordingly, intime, the charging time of the battery cell increases.

The charging technique disclosed in the '014 patent also takes intoaccount the voltage across various battery circuit elements. Inparticular, the voltage drop across the battery circuit elements ismonitored. During a normal charging condition, a “full” charging currentis applied to the battery. If the monitored voltage drop exceeds apredetermined value, the charging current to the battery is reduced to alow level for a predetermined time. After the predetermined time periodhas elapsed, the “full” charging current is restored to the battery.Unfortunately when the battery is being charged at a reduced currentlevel, the amount of time required to fully charge the battery isincreased.

U.S. Pat. No. 6,002,237, assigned to the same assignee as the assigneeof the present invention, discloses a rapid charging method for charginga lithium ion battery cell that also takes into account the voltage dropacross the external battery contacts as well as the other batterycircuit elements. The battery charging circuit disclosed in the '237patent monitors the voltage drop across the various circuit elements inthe battery circuit, as well as the battery cell voltage, to make surethat the maximum cell voltage is not exceeded. The battery cell voltageis measured by periodically interrupting the charging current flow tothe battery cell and taking a voltage measurement. The potentialdifference between the battery cell voltage (i.e. open circuit voltage)and the battery circuit elements (i.e. closed-circuit voltage) isperiodically determined. This potential difference represents thevoltage drop across various elements in the battery circuit. Thispotential difference is then used to adjust the charging voltage to thebattery during a constant current mode. By adjusting the chargingvoltage during a constant current mode, the charging current increases,thus decreasing the time for charging the lithium ion battery.

The system disclosed in the '237 patent requires periodic measurement ofthe potential difference between the battery cell voltage and theclosed-circuit voltage, which includes the battery cell voltage as wellas the potential drop across the battery circuit elements. Although thesystem disclosed in the '237 patent provides a rapid charge methodologyfor lithium ion batteries, the intent of the system disclosed in the'237 patent is to reduce the charging time by increasing the chargingcurrent once the battery cell voltage starts to increase and then reducethe charging current if no compensation for the increasing battery cellvoltage Vb takes place. Unfortunately, the battery charging circuitdisclosed in the '237 patent does not take into account the changes inthe series resistance values in the circuit over time and thus theeffectiveness of the circuit to rapidly charge a lithium ion batterywanes over time as the resistance of the series resistance elementsincreases.

There is a need to further reduce the charging time of such lithium ionbatteries.

SUMMARY OF THE INVENTION

Briefly, the present invention relates to a rapid charging circuit for alithium ion battery. The battery charging circuit compensates for thevoltage drop across the various resistance elements in the circuit plusan additional anticipated resistance to account for the increase inresistance of the series resistance elements over time, due to, forexample, oxidation of the external battery contacts, used to connect thebattery cell to the charging circuit. As such, the battery chargingcircuit is able to provide a maximum and constant current in order torapidly charge the battery cell over the expected change in theresistance of the battery charging circuit. In order to safely chargethe battery, the battery charger in accordance with the presentinvention periodically monitors the open-circuit voltage of the batterycell and reduces the charging voltage to the maximum voltage recommendedby the battery cell manufacturer, such as 4.2 volts DC, when the batterycell voltage reaches a predetermined value.

DESCRIPTION OF THE DRAWING

These and other advantages of the present invention will be readilyunderstood with reference to the following specification and attacheddrawing, wherein:

FIG. 1 is an exemplary characteristic charging curve for a lithium ionbattery in which the battery circuit has an additional series resistanceof zero ohms.

FIG. 2 is an exemplary characteristic charging curve for a lithium ionbattery in which the battery circuit has an additional series resistanceof one ohm.

FIG. 3 is similar to FIG. 1, but for a battery circuit that has anadditional series resistance of two ohms.

FIG. 4 is a block diagram of an exemplary battery charger circuit inaccordance with the present invention.

FIG. 5 is a schematic diagram of the exemplary battery charger circuitillustrated in FIG. 4.

FIG. 6 is an exemplary timing diagram illustrating the periodicmeasurement of the battery cell voltage.

FIG. 7 is an exemplary flowchart for the battery charger in accordancewith the present invention.

FIG. 8 is an exemplary flowchart for the periodic measurement of thebattery cell voltage.

DETAILED DESCRIPTION

The present invention relates to a battery charger circuit and, inparticular, a battery charger circuit for rapidly charging lithium ionbattery cells. As is known by those ordinarily skilled in the art, suchlithium ion batteries are charged in a constant current mode and in aconstant voltage mode. In accordance with an important aspect of theinvention, the battery charger circuit provides and maintains a constantand maximum charging current to the battery cell which not onlycompensates for the initial resistance of the series resistance elementsbut also the anticipated increase in resistance of the circuit overtime, foe example due to oxidation of the battery cell contacts toprovide rapid charging over the anticipated resistance change of thecircuit. By taking into account the maximum anticipated resistances inthe battery charging circuit, the battery charger circuit is configuredto provide a charging voltage level, that is able to provide a maximumand constant charging current to rapidly charge the battery cell overthe anticipated resistance range of the series elements. In order toprotect the battery cell from exceeding its maximum recommended batterycell voltage during charging, the battery cell voltage is periodicallymonitored by turning off the charging current and measuring the batterycell voltage. When the system detects that the battery cell voltage hascharged up to a predetermined level, at or slightly below, for example,the maximum charging voltage recommended by the battery cellmanufacturer, for example, 4.175 volts DC, the charging voltage isreduced to the maximum recommended charging voltage to avoidover-charging the battery cell. The battery charging is thereafterterminated when the charging current drops below a predetermined value.

FIGS. 1-3 illustrate exemplary battery charging curves for an exemplarybattery charging circuit for three different conditions as describedbelow. In particular, FIG. 1 illustrates an exemplary battery chargingcurve for a condition when only the initial resistance of the batterycharging circuit is considered and 0 ohms of additional resistance. FIG.2 is similar to FIG. 1, but for a design condition which includes anadditional resistance of 1 ohm., to compensate for the change inresistance over time, primarily due, for example, oxidation of thebattery external contacts. FIG. 3 is similar but for a condition whenthe resistance of the battery charging circuit includes an additionalresistance of 2 ohms, which, in this example is not figured into thedesign, for example, due to cost considerations.

As shown in FIG. 1, the curve 10 refers to the open circuit battery cellvoltage while the curve 12 relates to the charging voltage. As will bediscussed in more detail below, the battery cell voltage (also known asopen cell voltage or OCV) is measured by interrupting the current to thebattery circuit and simply measuring the battery cell voltage. The curve14 illustrates the charging current. The charging voltage, curve 12(also known as the closed circuit voltage or CCV) equals the sum of thevoltage drop across the resistances in the battery circuit plus thebattery cell voltage. The charging voltage is measured with the currentflowing through the circuit. During this ideal condition, the chargingvoltage 12 is selected at a level that is slightly higher than themaximum recommended cell voltage so that charging current is constantand at a maximum level as shown. The charging voltage 12 is the closedcircuit voltage and is the sum of the battery cell voltage plus thevoltage drops due to the various resistance elements in the batterycharging circuit. Thus, the charging voltage 12 will track the batterycell voltage 10 as shown. When the battery cell voltage 10 reaches apredetermined value, for example, a value at or near the maximumrecommended battery cell voltage, as indicated by the point 16, thecharger switches to constant potential mode and the charging voltage 12is reduced to the maximum recommended battery cell charging voltage. Thecharging current 14 then begins to taper, as indicated by the point 18,and is terminated when the charging current 14 drops below apredetermined level.

FIG. 2 illustrates a condition where the resistance of the batterycharging circuit is increased by 1 ohm., for example, due to theoxidation of the battery cell contacts. In accordance with the presentinvention, an additional resistance due to the change in resistance overtime is considered. As such, as shown in FIG. 2, the maximum chargingcurrent will be the same as the condition illustrated in FIG. 1. Inparticular, the battery cell voltage is illustrated by the curve 20 Thecurve 22 represents the charging voltage. The charging voltage The arrow24 represents the potential difference between the closed-circuitvoltage (CCV) and the open-circuit voltage (OCV). This potentialdifference represents the voltage drop across the various circuitelements and in this case, the additional 1 ohm. As indicated in FIG. 2,the CCV curve 22 is higher than the charging voltage for the conditionillustrated in FIG. 1, in order to compensate for the additional 1 ohmof resistance and at the same time provide the maximum charging currentof I_(max) as in FIG. 1. As such, the battery charger circuit isconfigured to provide and maintain I_(max) for the anticipatedresistance change of the battery charging circuit-in this case 1 ohm. Asshown, the charging current 26 remains constant during the constantcurrent mode to charge the battery cell as rapidly as possible. In orderto prevent the battery cell from being charged at a voltage that exceedsa predetermined level as discussed above the system periodically checksthe battery cell voltage to determine if the battery cell voltagereaches the predetermined value. When the battery cell reaches thepredetermined value as indicated by the point 28, the charging voltageis reduced to a value at or below the maximum recommended voltage, asindicated by the segment 30 until the battery cell is fully charged;i.e. the charging current drops below a minimal value, such as 100-200mA. When the charging current drops to this range, for example, asindicated by the point 32, the charging is considered complete and thecharger voltage is turned off.

FIG. 3 is similar to FIG. 2, but illustrates a condition that isdesigned for in this example; a condition when the resistance of thecharging circuit is increased by 2 ohms. In this example, the batterycell voltage is identified with the reference numeral 36, while thecharging voltage is identified with the reference numeral 38. Thecharging current is illustrated with the curve 40. In this example,since the charging circuit was designed for a anticipated increasedcharging circuit resistance of 1 ohm, the maximum available chargingvoltage due to design based on cost is a little over 5.0 volts DC. Asshown, during this condition, the battery charger can still charge thebattery cell but not as rapidly when the circuit resistance was lower,as indicated in FIGS. 1 and 2. For example, during this condition, thetotal time to charge the battery cell is about 3.8 hours, as opposed tothe conditions illustrated in FIGS. 1 and 2 which illustrate a totalcharging time of about 2.5 hours. When the battery cell voltage reachesthe maximum predetermined voltage, as indicated by the point 42, thecharging voltage is reduced to the maximum recommended cell chargingvoltage. The system then turns off as discussed above when the chargingcurrent reaches a predetermined value, as indicated by the point 43. Asshown by point 40, during this condition, with an additional seriesresistance of 2 ohms, the battery charger is unable to maintain aconstant current and tapers during the constant current mode.

An exemplary block diagram for a charger in accordance with the presentinvention is illustrated in FIG. 4 and is generally identified with thereference numeral 49. The charger includes three main components: aDC-DC converter 51, configured to receive an input voltage of, forexample, 9-16 volts DC, a voltage regulator 53 and a microprocessor unit(MCU) 55. In general, the voltage regulator 53 is used to provide a 3volt reference for the MCU 55 along line 54. The DC/DC Converter 51generates the voltage and current to be delivered to its positive outputterminal 57 for constant current and constant voltage charging of abattery cell to be charged. A negative terminal 59 is connected tosystem ground by way of a current sense resistor 61. The MCU 55generates a pulse width modulated (PWM) signal along line 68. This PWMsignal may be filtered by a filter circuit 69 and is used to control thevoltage output level of the DC/DC Converter 51.

The output of the DC/DC Converter 51 to the positive terminal 57 isunder the control of the MCU 55 as a function of one or more batterycharacteristics. More specifically, the MCU 55 monitors three batterycharacteristics: battery voltage; charging current; and batterytemperature The open circuit voltage of the output terminal 57 ismonitored via line 60 during an open circuit condition when no chargingcurrent is being delivered to the positive output terminal 57 by simplyturning off the DC/DC Converter 51 and measuring the voltage. Since nocurrent flows in the battery circuit during this condition, the measuredvoltage on line 60 represents the open circuit battery cell voltage. Thecharging current to the battery cell is sensed by way of line 63. Inparticular, a current sense resistor 61 that is serially connectedbetween the negative charger terminal 59 and system ground provides avoltage signal on line 63 that is representative of the charging currentapplied to the battery cell. As mentioned above, the charging current issensed in order to maintain a constant current during a constant currentmode and detect when charging is complete. The battery temperature issensed by a thermistor 67 that is connected to the MCU 55 by way of aline 65. The voltage output of the thermistor is representative of thebattery temperature.

In particular, in accordance with an important aspect of the invention,the output level of the DC/DC Converter 51 is designed to provide acharging voltage, typically greater than the maximum recommended batterycell voltage, which compensates for the maximum expected voltage drop ofthe various resistance elements in the battery charging circuit toprovide and maintain a maximum charging current to the battery celluntil the battery cell is charged to its maximum recommended value, thusreducing the charging time of the battery. The system periodicallymonitors the battery cell voltage to determine when the battery cellvoltage is either at or near its maximum recommended voltage (i.e. 4.2volts). When the battery cell reaches a value around its maximumrecommended value, the battery charging voltage is reduced to a value ator below the maximum recommended battery cell voltage by way of afeedback loop 70 in order to safely charge the battery cell. After thecharging voltage is reduced, the system monitors the charging currentand terminates charging when the charging current drops below apredetermined value.

The circuit illustrated in FIG. 4 may be used in conjunction with acigarette lighter adapter (CLA) for charging various electronic devices.The principles of the present invention are also applicable to ACchargers, which include a switched mode power supply or AC/DC adapter.

A more detailed schematic is illustrated in FIG. 5. In particular, thereference voltage regulator 53 may include a 3 terminal programmableshunt regulator U2, for example a Fairchild Model No. TL431, atransistor Q1 and a plurality of resistors R13, R14, R15 and a capacitorC8. The input power is provided by an unregulated source of DC voltagein the range of, for example, 9-16 volts DC, from an AC/DC power supplyor an automotive electrical system, at a positive input terminal 71. Anegative input terminal 73 is connected to system ground. As mentionedabove the reference voltage regulator is used to provide a 3-voltreference to the MCU 55. The transistor Q1 is a regulation transistorthat is under the control of the shunt regulator U2. The emitter of thetransistor Q1 is attached to a voltage supply terminal Vcc of amicroprocessor U1 that forms part of the MCU 55. The collector terminalof the transistor Q1 is connected to the positive input terminal 71,while the base is connected to the shunt voltage regulator U1. Aresistor R13, connected between the collector and base of the transistorQ1 is used to provide bias current for the transistor Q1 and the shuntregulator U2. The K and A terminals of the shunt regulator U2 areconnected between the base of the transistor Q1 and system ground. Avoltage divider consisting of the serially coupled resistors R14 and R15is connected between the emitter terminal of the transistor Q1 andground. A node 74 defined between the resistors R14 and R15 is connectedto an R terminal of the shunt regulator U2. The R pin of the shuntregulator U2 is used for feedback control, which in turn controls the Kpin to maintain a constant voltage at the emitter of the transistor Q1.The output of the regulator 53 is 2.495*R15/(R14+R15)=3 volts. A filtercapacitor C8 may be connected between the emitter terminal of thetransistor Q1 and ground to absorb the noise.

The DC/DC converter 51 may be a buck converter which includes aswitching element, for example, a Fairchild Model No. 34063, an inductorL2, a diode D1, a plurality of capacitors C3, C5, C16 and variousresistors R4, R6 and R7. The resistors R6 and R7 form a voltage dividerand are used to sense the output voltage of the DC/DC converter 51. Thisoutput voltage is fed back to pin 5 of U3. The buck converter U3controls its output switching pulse on pin 2 in order to maintain aconstant output voltage based upon the feedback voltage applied to pin5. The inductor L2, freewheeling diode D1, capacitor C5 along with theresistors R6 and R7 form an energy storage circuit. In particular, theinductor L2, coupled between the output of the buck converter U3 (i.e.pin 2) and the output terminal 57, stores the output pulse energy fromthe buck converter U3 and provides a stable output current. The diodeD1, for example, a Schottky diode, provides current to the inductor L2during low periods of the output pulse from the buck converter U3.During such periods, current flows through the inductor L2, the freewheeling diode D1 and the resistors R6 and R7. A capacitor C5 may beconnected across the resistors R6 and R7 to absorb the switching noiseand smooth out any ripple voltage and ripple current.

The capacitor C16 is a timing capacitor and is used to set the timing ofan internal oscillator within the buck converter U3. An electrolyticcapacitor C3 may be connected across the input terminals for noisefiltering and to stabilize the voltage applied to the input terminals 71and 73. A sensing resistor R4 may be connected between the inputterminal 71 and pins 1 and 8 of the buck converter U3. The resistor R4is used to sense the input current to the buck converter U3. If theinput current to the buck converter U3 is too high, for example, due toswitching, the buck converter U3 switches off its internal powertransistor.

The MCU 55 is used to control the output voltage of the buck converterU3. The MCU 55 includes a microprocessor U1, for example, an Atmel ModelTINY13, a plurality of resistors R8, R9, R11, R16, R18, R19, R20 andR21, a thermistor RT, a capacitor C17 and an LED.

Three inputs are applied to the microprocessor U1: charging current,battery voltage and battery temperature. The thermistor RT is used tosense the battery temperature and is applied via line 65 to pin 1 of themicroprocessor U1. The resistor R16 bias voltage for the thermistor RT.The resistor R11 is used to sense the charging current to the battery asdiscussed above and is applied to pin 3 of the microprocessor U1 vialine 63. The resistors R8 and R9 form a voltage divider and are used toprovide the microprocessor U1 with the battery cell voltage and the buckconverter U3 output voltage. A node defined between the serially coupledresistors R8 and R9 is applied to pin 2 of the microprocessor via line60.

The microprocessor U1 monitors the charging current, battery voltage andbattery temperature and generates a feedback signal that is used tocontrol the output voltage of the buck converter U3. In particular, themicroprocessor U1 periodically monitors the open circuit voltage byturning off the current from the buck converter U3 and measuring thevoltage. Since no current is flowing in the circuit, the open circuitvoltage represents the battery cell voltage. This battery cell voltageis checked to see if it is at or below the maximum predetermined batterycell voltage. In particular, the microprocessor adjusts the output pulsewidth of its PWM on its pin 6, which is applied to pin 5 of the buckconverter U3 by way of a PWM filter formed from the resistors R19, R20and R21 and the capacitor C17, which converts the pulse to a constant DCvalue that is proportional to the width of the pulse. The magnitude ofthe DC value is then applied to pin 5 of the buck converter U3, whichcontrols the output voltage of the buck converter U3.

In order to detect the battery cell voltage, the charging current is setto zero and the battery cell voltage is measured at the + outputterminal 57 (FIG. 5). In particular. The battery cell voltage is appliedto the V_(sense) terminal of the microprocessor U1 by way of the voltagedivider formed from the resistors R8/R9. In order to measure the batterycell voltage, the microprocessor U1 sets a “PWM Control Output” signal(i.e. pin 5) on the DC-DC Converter 51 high. The PWM Control Outputsignal is compared with an internal reference voltage. When the PWMOutput Control signal is high, its magnitude will be greater than theinternal reference voltage which causes the DC-DC Converter 51 to turnoff its output, thus turning off the charging current to the battery.When the charging current is off, the battery cell voltage is measuredas discussed above.

The battery cell voltage is periodically measured as illustrated in FIG.6. Referring to FIG. 6, the battery cell voltage is periodicallymeasured as indicated by the pulses, identified with the referencenumerals 82, 84, 86, and 88. As mentioned above, during periodicmeasurement of the battery cell voltage, the charging current isinterrupted. More specifically, the waveform 90 illustrates the chargingcurrent during a portion of a charging cycle while the waveform 92illustrates the charging voltage. As shown, the charging current ismaintained at a value IC mA during a portion of the charging cycle asindicated by the segments 94, 96, and 98 of the waveform 90. Duringperiods when the battery cell voltage is measured, the charging currentis dropped to zero mA, as indicated by the segments 100 and 102 for atime period, for example, t₂. Similarly, the segments 104, 106, and 108represent portions of the charging cycle when the charging voltage(i.e., closed circuit voltage) is applied to the battery cell. Duringthe time period t₂, the voltage of the battery cell is measured asindicated by the line segments 110 and 112. As shown, the time period t₁represents the total time for a sampling cycle. The time period t₂represents a portion of the sampling cycle as shown.

Low Ripple Output

In accordance with another important of the invention, the lithium ion(LiIon) battery charging circuit is configured to provide a low rippleoutput. Other known battery charging circuits, such as the NiMH batterycharger topology disclosed in commonly owned pending patent applicationSer. No. 10/897,285, filed on Jul. 22, 2004, uses a microprocessor withan on board pulse width modulator (PWM) together with feedback providedby measurement of current (constant current) or battery voltage(constant voltage) to drive a transistor/FET switch of a buck regulator.Because of the relatively slow measurement acquisition time for currentor voltage feedback and the overhead of executing code and loading newvalues for the pulse width modulator registers, a phase difference orlag time exits between the measurement and driving of the buck regulatortransistor. If the input voltage varies periodically to the buckregulator, a component of this voltage ripple passes through the buckregulator. This ripple component is typically transparent for NiMHbatteries but can be too high for optimum charging of LiIon batteries.

Lithium Ion charging systems require very accurate voltage measurementsand control. The standard buck regulator as described in the aboveidentified patent application is inadequate for LiIon voltage chargingdue to the large voltage swings caused by the phase error. To correctthis problem for LiIon charging, a secondary control loop is createdusing the buck controller U3 configured to operate typically between 50kHz to 100 kHz as set by the capacitor C16. The buck regulator's controlnode is set using the divider formed by the R6 and R7. This node isconnected to the inverting side of the internal comparator, pin 5, onthe buck regulator U3. The non-inverting side is tied to the internal1.25 volt reference. As the input to pin 5, when the inverting side ofthe comparator exceeds the 1.25 volt threshold, the duty cycle of thebuck regulator's internal PWM will decrease, causing the divider nodevoltage formed by the resistors R6 and R7 to maintain 1.25 volts. Thishigh frequency PWM feedback effectively regulates the output of the buckregulator in a fast loop and reduces ripple to acceptable levels forLiIon charging.

The primary control loop is established by the microprocessor U1. Inparticular, the microprocessor U1 controls the control node of the buckregulator U3 by modulating the duty cycle to the RC network formed bythe resistor R20 and the capacitor C17. The time constant of the RCcircuit formed by the resistor R20 and the capacitor C17 must be muchgreater the modulation period. The voltage created by the RC circuitcreates a potential difference across the resistor R21. This voltagedrop sinks current through the resistor R6 dropping the reference nodeto the buck regulator U3. The charger can run in either constant currentmode or constant voltage mode under software control and will adjust theRC voltage appropriately. In a constant current mode, the modulationperiod is a function of the voltage measured at the resistor R11. In aconstant voltage mode, the modulation period is determined by thevoltage measured through the voltage divider formed by the resistors R8and R9. The advantage to this approach is that the primary loop is nowresponsible for only setting a reference level to the buck regulator U3.The primary loop speed is not critical and is compliant with thecapability of the microprocessor U1. With this system, themicroprocessor U1 is now capable of managing the mode, constant currentor constant voltage, and the magnitude of the output levels via softwarecontrol and the resulting system output is tightly regulated andprovides accurate, low ripple voltage needed to properly charge LiIonsystems at low cost.

One of the functions of the microprocessor U1 is to make criticaldecisions as to the state of the battery cell. During start up, it iscritical that the buck controller U3 be held in the off state to preventcurrent flow. Since the microprocessor power up sequence can takeseveral milliseconds and buck regulator's sequence is on the order ofmicroseconds the resistors R19, R20 and R21 are used as pullup resistorsto the microprocessor U1 supply voltage Vcc. During the microprocessorstart up period, pin#6 of the buck controller U3 stays in highresistance mode. The resistors R19, R20 and R21 form a voltage dividerwith the resistor R7 that drives the inverting side of the buckregulator's comparator to 1.43 volts which is well beyond the 1.25 voltregulation point of the buck regulator's reference. This effectivelyshuts down the buck regulator U3 until the microprocessor U1 can startup and take overall control of the system.

Flowchart

A flowchart for the exemplary battery charger is illustrated in FIG. 7.A flow chart for measuring the battery cell voltage is illustrated inFIG. 8.

Referring first to FIG. 7, the system first checks in step 120 todetermine if a battery pack is inserted. The battery pack is sensed bymeasuring the voltage at the output terminal 57 by way of the voltagedivider resistors R8/R9. When no battery pack is connected to the outputterminals 57 and 59, no voltage will be present. When a battery pack isconnected between the output terminals 57 and 59, the microprocessor U1will measure a voltage across the output terminals 57, 59. If no batterypack is connected across the output terminals 57/59, the system loopsback to step 120 waiting for a battery pack to be installed. Once thesystem detects that a battery pack has been connected to the outputterminals 57, 59, the system checks in step 122 whether the temperatureof the battery pack is at a temperature extreme, such as less than −7°C. or greater than 46° C. The temperature is sensed by a thermistor RT67 (FIG. 5). If the temperature of the battery cell is greater than 46°C. or less than −7° C., the system loops back to step 122 and repeatsthe temperature check. During this condition, it is assumed that thebattery cell is either too hot or too cold for charging. Alternatively,if the battery cell is not too hot or too cold, the system proceeds tostep 124 and determines whether the battery cell voltage is greater thana predetermined value, for example, 4.35 volts. If so, the systemassumes a fault and indicates a fault mode in step 126. However, if thebattery cell voltage is less than 4.35 volts, the system checks in step126 to determine whether the battery cell is fully charged. As discussedabove, the full-charge condition of the battery cell is assumed to occurwhen the charging current falls below a nominal value, for example, 100mA-200 mA. If so, the system proceeds to step 128 and indicates that thecharge is complete. If the charging current is greater than the valueindicated above, the system again checks the battery cell voltage todetermine how close it is to the maximum recommended voltage of 4.2volts. If the battery cell voltage is less than, for example, 4.175volts, the system continues with a constant current mode in step 132 andloops back to step 130 until the battery cell voltage reaches about4.175 volts. Once the battery cell voltage reaches 4.175 volts, thecharging voltage is reduced in step 134 to the maximum recommendedvoltage of about 4.2 volts to avoid overcharging the battery in step134. After the charging voltage is lowered to 4.2 volts, the systemchecks the value of the charging current in step 136. If the chargingcurrent is less than a predetermined value, for example, 90 mA, thesystem assumes that the charge is complete and proceeds to step 128.Alternatively, if the charging current is greater than mA, the systeminitiates a timer in step 138 and continues to charge at 4.2 volts untilthe timer times out. Once the tinier times out, the system proceeds tostep 128 and assumes that the charge is complete.

FIG. 8 is an exemplary flow chart that illustrates the steps forperiodically determining the battery cell voltage as indicated in steps124 and 130 of FIG. 7. As indicated in step 140, a sampling timing cyclet₁ is initiated on the falling edge of a current pulse 82. In otherwords, the timer t₁ is initiated when the charging current initiallygoes to 0 mA. As indicated in FIG. 6, at the end of a sampling cycle(i.e., at t₁), the charging current is turned off as indicated by thesegments 100, 102 (FIG. 6) by setting the PWM output control pin 5 onthe DC-DC converter 51 to a high level, as indicated in step 142. Thehigh level is maintained for a time period t₂ as indicated by the pulses82, 84 (FIG. 6) in step 144. During this time period t₂, the batterycell voltage is measured in step 146. The system returns to the mainloop in step 148.

Obviously, many modifications and variations of the present inventionare possible in light of the above teachings. Thus, it is to beunderstood that, within the scope of the appended claims, the inventionmay be practiced otherwise than is specifically described above.

1. A method for fast charging a lithium ion battery cell having amaximum recommended battery cell voltage, the method comprising: (a)providing a charging current during a constant current mode of operationduring which a first charging voltage and a constant current are appliedto said battery cell; (b) periodically measuring the battery cell opencircuit voltage to determine if it has reached a predetermined value;(c) maintaining said constant current mode of operation until saidbattery cell open circuit voltage reaches a predetermined value lessthan said maximum recommended battery cell voltage; (d) switching to aconstant voltage mode of operation when said open circuit battery cellvoltage reaches said predetermined value, wherein in said constantvoltage mode of operation a constant second voltage is applied to saidbattery cell during said constant voltage mode of operation; (e)monitoring the charging current supplied to said battery cell duringsaid constant voltage mode of operation; and (e) terminating chargingwhen said charging current during said constant voltage mode ofoperation drops below a predetermined value.
 2. The method as recited inclaim 1, wherein step (a) comprises: (a) providing a first chargingvoltage that has been selected to compensate for the maximum anticipatedresistance in a battery charging circuit coupled to the battery cellincluding an internal battery cell resistance, battery terminals, andwire connected to the circuit.
 3. The method as recited in claim 1,wherein step (b) comprises turning off the charging current to thebattery cell while the battery cell voltage is being measured.
 4. Themethod as recited in claim 3, wherein step (b) includes the step ofperiodically turning off the charging current to the battery cell. 5.The method as recited in claim 3, wherein step (b) includes the step ofturning off the charging current to the battery cell for a predeterminedtime period t and taking one or more battery cell voltage measurements.